Anesthesia for Intraoperative Neurophysiologic Monitoring of the Spinal Cord

Abstract

Intraoperative neurophysiologic monitoring (INM) using somatosensory and motor evoked potentials (MEPs) has become popular to reduce neural risk and to improve intraoperative surgical decision making. Intraoperative neurophysiologic monitoring is affected by the choice and management of the anesthetic agents chosen. Because inhalational and intravenous anesthetic agents have effects on neural synaptic and axonal functional activities, the anesthetic effect on any given response will depend on the pathway affected and the mechanism of action of the anesthetic agent (i.e., direct inhibition or indirect effects based on changes in the balance of inhibitory or excitatory inputs). In general, responses that are more highly dependent on synaptic function will have more marked reductions in amplitude and increases in latency as a result of the synaptic effects of inhalational anesthetic agents and similar effects at higher doses of intravenous agents. Hence, recording cortical somatosensory evoked potentials and myogenic MEPs requires critical anesthetic choices for INM. The management of the physiologic milieu is also important as central nervous system blood flow, intracranial pressure, blood rheology, temperature, and arterial carbon dioxide partial pressure produce alterations in the responses consistent with the support of neural functioning. Finally, the management of pharmacologic neuromuscular blockade is critical to myogenic MEP recording in which some blockade may be desirable for surgery but excessive blockade may eliminate responses. A close working relationship of the monitoring team, the anesthesiologist, and the surgeon is key to the successful conduct and interpretation of INM.

Comparing the effects of vecuronium and cisatracurium on electrophysiologic monitoring during neurosurgery: a randomized controlled study

“…Heyer [2] suggested that successful monitoring has been accomplished at T1 between 5% and 50% of baseline. Usually, the variability in MEP amplitude decreases with lower NMB levels [12], and we presumed that T1/Tc 50% would allow for successful MEP monitoring and detection of any inter-group variability differences; in fact, with the NMB levels set at T1/Tc 50%, MEP monitoring was successful in this study.…”

“…Some neuromuscular blockade (NMB) drugs significantly influence MEP monitoring during anesthesia [2,3]. However, if no NMB drug is used, higher levels of anesthetics are required to avoid the risk of spontaneous movement during surgery.…”

“…One anesthetic technique for ensuring successful MEP monitoring with absolute immobility is to maintain partial NMB status during monitoring. Continuous intravenous NMB drug infusion is occasionally used to achieve partial NMB status [1,2,6,7]; in that situation, maintaining strictly steady NMB status is essential to ensure stable MEP monitoring.…”

Intraoperative neurophysiological monitoring for the anaesthetist

“…The more synapses in the neurological pathway that is being monitored, the more marked the effect on latency and amplitude of the EPs. 5 Generally, anaesthetic agents will cause an increase in latency and a decrease in the amplitude of EPs.…”

“…6 SSEPs are intermediate in sensitivity and depend on the type of agents used. 5 MEPs are dependent on a functioning NMJ, and may be totally abolished during the use of NMAs. Transcranial MEPs in children can be significantly depressed, with the use of halogenated agents at lower concentrations than those in adults.…”